Games Primates Play: An Undercover Investigation of the Evolution and Economics of Human Relationships

by Dario Maestripieri

  Two evolutionary psychologists, Robert Kurzban and Jason Weeden, analyzed behavioral and questionnaire data from 10,526 participants in HurryDate sessions.7 They discovered that among both men and women, some individuals are in high demand (their mate value is high) and others are not. The characteristics associated with high mate value in this situation were almost exclusively physically observable attributes such as attractiveness, slenderness, height, and youth, whereas harder-to-observe attributes such as education, religion, sociosexuality, or views on children didn’t matter at all.

  Consistent with the idea that people are aware of their participation in a market and know how to navigate it, high-value individuals were picky and ended up selecting only individuals who had high mate value as well. Conversely, less desirable men and women were also less selective about their partners. For example, heavier women agreed to a relatively high proportion of potential dates, as did men who were either heavy or very thin. In this study, desirability was directly related to physical attractiveness for both men and women. Clearly, given the logistics of speed-dating, it is not surprising that in this context, physical attractiveness is the most important factor for partner selection. Other studies investigating online dating, however, have found that the best predictors of the number of opposite-sex emails received is physical attractiveness in the profile photo for women and income for men. Good-looking women and high-income men get the most emails.

  Biological Markets

  In nature, situations in which individuals cooperate with one another can be thought of as markets in which commodities of similar or different value are exchanged according to the laws of supply and demand, usually through advertisement and bartering. We call these situations biological markets to distinguish them from human markets in which money is used as currency. Many biological markets involve two distinct classes of traders: in the mating and reproduction market the traders are males and females, and in the alliance market they are high-ranking and low-ranking individuals. Traders can also be animals of different species, such as cleaner fish and their customers, or plants that need to be pollinated by insects and the insects that pollinate them.

  In the mating market, males offer the following commodities to females: sperm to fertilize the females’ eggs, good-quality genetic material that will make offspring healthy and attractive, and help with child-rearing. In some animal species, males also offer territories that contain food, nests in which females can lay their eggs, or so-called nuptial gifts involving food that can be consumed by females before or during mating. Females, in turn, offer eggs to be fertilized by males, bodies in which the embryos can grow, and the ability to take care of the babies. In primate markets, as we’ll see later, monkeys can trade grooming for other grooming, for sex, or for agonistic support.

  In biological markets, as in other kinds of markets, some traders possess commodities of higher value or quality than others do. In the case of territorial animals, for example, some males may have larger territories, or territories containing more food or better nests, than other males. Traders of one class can choose among traders of the other class in relation to the value of their commodity, but they must compete with members of their own class for access to preferred potential partners. Males can choose a mating partner among many different females, but must also compete with other males to mate with highly attractive females. Partner choice is important in a market because doing business with selected individuals who possess high-value commodities is generally more profitable (for animals, profit means increased survival and future reproduction) than doing business with a random individual. I emphasize the term choice because biological market theory assumes that commodities cannot be obtained through force but only with the consent of the trading partner. Similarly, competition with members of one’s own class does not generally involve aggression or intimidation. For example, males who compete with each other to mate with an attractive female cannot eliminate each other by force. Rather, traders in the same class compete by trying to outbid each other in the value of the commodity offered: they try to offer better products than their competitors. In turn, decisions to cooperate with a particular partner—partner choice—are based on the comparisons between the offers of different potential partners.

  The issue of comparing the offers of different bidders is of crucial importance in biological markets. When a female chooses to mate with a particular male on the basis of the territory he possesses, she must have the ability to directly assess the quality of the territory, to inspect the territories of different males, and to make comparisons among them. The sampling of individuals with different commodities can be a complicated, expensive, and time-consuming process. It is often simply too expensive to sample all bidders, and one can sample only a subset of prospective partners. Thus, when evolutionary biologists develop market models to predict the partner choice decisions made by particular individuals in particular situations, the costs of sampling and assessment are crucial variables because they determine whether the number of potential partners is high (when costs are low) or low (when costs are high).

  The accuracy of the strategy and procedures used to sample potential bidders and assess the quality of their commodities is also an important variable. In some biological markets, the value of the commodity can be assessed directly. In insect species in which males bring females a nuptial gift, the female can immediately assess the size and quality of the gift. In other markets, however, assessment is based on an advertisement that is supposed to represent the quality of a commodity. Male birds often advertise their good health, strength, and high social status through brightly colored stripes or spots on their plumage. Where there is advertisement, however, there is also potential for false information. Just as people who watch TV commercials cannot tell whether the qualities of the product being advertised are real, when females assess the quality of potential mating partners indirectly, through signals produced by the males themselves, they can’t be sure the males are being honest. Thus, among traders there may be individuals who pretend to offer a particular high-quality commodity but then can’t deliver on their promise—so-called free riders. I already discussed the issue of honest and deceitful signals when I mentioned the Handicap Principle in Chapter 7. The use of signals to advertise commodities implies that when traders do business with each other, they not only inspect and compare the commodities of different potential partners but also communicate directly with them, engaging in bartering and bargaining.

  The exchange value of a commodity in a biological market is determined by the ratio between supply and demand, and this can vary over time. As we’ll see later, the value of a nest as a commodity that male birds offer to attract females may vary during the course of a year depending on how easy or difficult it is to build nests, how many males are able to do it at a particular point in time, and how urgently females need these nests to lay their eggs. Studies of biological markets have shown that the establishment of cooperative relationships between individuals, including partner choice, changes over time in relation to temporal fluctuations in the supply and demand of particular commodities.

  Another important characteristic of biological markets is that they are often skewed, which means that one commodity is in high demand and another one isn’t. This may happen because the members of one trading class outnumber the others, or simply because one commodity is abundant and another one is rare. Women’s eggs are in shorter supply than men’s sperm because women have only one mature egg per month until they reach menopause, while men can produce millions of sperm every day of their lives. Traders who possess a commodity that is high in demand become the choosing class and can find a partner easily, while traders with a commodity that is low in demand become the chosen class and have to outbid their class members to find partners. In the mating market, females can contribute to this process of competition by playing males off against one another and forcing them to increase their bids during courtship, such as by offe
ring food or services or engaging in risky behavior—or in the case of humans, simply spending a lot of money. Traders who can’t afford to make competitive bids are forced to take less favorable options and settle with less valuable partners.

  Let me now provide some examples from the animal kingdom that illustrate some of these general principles regulating “business” in biological markets.

  ANIMAL MATING MARKETS

  Primatologist Michael Gumert examined the mating market in wild long-tail macaques, a species similar to rhesus macaques that lives in the forests of Indonesia.8 These monkeys live in large groups with many females and many males. Females are fertile during four or five days in the middle of their menstrual cycle and advertise their estrus with sexual swellings. Normally, at any point in time, half the females in the group are pregnant or breast-feeding a young baby and so are neither fertile nor interested in sex. The menstrual cycles of the other females are usually not synchronized, so they don’t all become fertile at the same time. This means that when one female in the group is in estrus, her fertility is a valuable commodity that every male in the group wants. Males cannot sexually coerce the fertile female and cannot use force to prevent their male competitors from mating with her. Instead, they have to offer the fertile female another commodity and outbid their competitors to make sure that she will do business with them. This commodity is grooming. Receiving grooming increases one’s hygiene and reduces tension. It’s sort of like when a husband offers a back rub to his wife in hopes that she will consent to have sex. That macaques consider grooming a valuable commodity is suggested by studies showing that individuals often mutually groom each other and do so in a time-matched manner, and that when subordinates groom high-ranking individuals they obtain tolerance and support in return. Does grooming also work as payment for sex?

  Gumert observed that when there is a fertile female in the group, the males groom her much more than they groom the nonfertile females. He also observed that after a male grooms the fertile female, they often have sex. It turned out that male grooming before sex lasts longer than grooming not followed by sex, and that a female is more likely to have sex with a male after this male has groomed her for a while than if he just sits there and does nothing. So it seemed that being groomed by a male for a while puts the female in the right mood for sex. One interpretation of this is that males use grooming to pay fertile females for sex. In contrast, fertile females don’t make any grooming payments to males in order to have sex with them. All fertile females have to do to get some romantic attention from males is to look pretty with their sexual swellings.

  Gumert also noticed that not all males pay the same grooming price for sex, and not all females receive the same compensation for their availability. High-ranking males are generally more attractive than low-ranking ones as mating partners because they can provide better protection from other monkeys to females and their offspring. High-ranking females are more attractive as mating partners because they are generally healthier and more fertile than low-ranking ones. As would be expected in a mating market, the value of individuals influences the commodities they are able to obtain as well as the price they pay for them. High-ranking males groom fertile females less but mate with them more than low-ranking males do. Given that low-ranking males are less attractive, they have to work harder at gaining their favors. High-ranking females mate with high-ranking males more often and receive higher grooming payments from males for the same amount of sex as low-ranking females do. Finally, in accordance with the law of supply and demand, if there is only one fertile female and many males in the group at a particular time, the grooming payment made by the guy who gets lucky is very high. If there are more fertile females at the same time in the group, the grooming payment received by each fertile female is lower.

  As you can imagine, Gumert’s article, aptly titled “Payment for Sex in a Macaque Mating Market,” made a big splash with the media when it came out in 2007. Newspapers, magazines, and Internet news sites played around with headlines that more or less explicitly hinted at the discovery of monkey prostitution. Spike TV sent a crew to my office to interview me about this article, but I totally “choked” in front of the camera and I don’t think the interview ever aired.

  Mating markets are common in birds. A good example is the mating market of the red bishop, a colonial weaverbird living in southern Africa, which has been studied by German biologist Markus Metz and his colleagues.9 Red bishop males mate with many different females, which they try to attract by building several nests within their territories. Unlike the many other bird species in which males and females take care of their chicks together, red bishop females incubate eggs and feed the chicks entirely on their own. Females, however, don’t build nests; nests are the commodities offered by males when they try to entice females to do business with them.

  Females sample many males before choosing a mating partner, and they base their choice entirely on the quality of the nests. Some males build many nests, others only a few; some nests are good, others just okay. The males have adjacent territories, so it’s easy for females to hop around from one territory to the next to examine the merchandise; in other words, the costs of sampling are low, and females can afford to examine many nests before choosing one. There are always more nests on the market than females who are looking for them, and as is always the case when the supply is greater than the demand, females are very choosy about nests, while males try to outbid each other by offering better nests than their neighbors. When the female demand for nests increases at particular times of the year—because greater availability of food allows for more intense breeding activity—males build nests more quickly. Since females prefer fresh new nests that have been built within the past week over those that have been on the market for a long time, all males are under pressure to put new nests out on the market.

  Choosy females inspect many different nests carefully before settling on a brand-new one in which they lay their eggs. Females become more likely to accept older nests when their own market value decreases, that is, when there are many females searching for nests but only a few nests are available. Therefore, partner choice in the red bishop mating market is regulated by the value of nests and other commodities traded by males and females, which in turn depends on temporal fluctuations in their supply and demand.

  Mating markets similar to that of the red bishop are also found in insects, but instead of nests, males offer females food to convince them to mate.10 In scorpion flies, males offer females smaller insects as nuptial gifts, and females choose their mating partners not only by allowing some males to mate with them but by refusing or interrupting mating with others. Cheating by males is difficult since females can assess the size and quality of the gift immediately. The larger the gift, the higher the probability of successful mating. Studies of scorpion flies have shown a clear market effect on male-female business transactions: females reject males with small gifts when many males offering food are available, but they accept gifts of any size when there are only a few offers on the market. In some species of spiders, males offer themselves as food to females—the ultimate sacrifice—in order to mate with them. When a female accepts a particular male as a mating partner—which means that she is either horny or hungry, or both—she will often start chewing on the male’s head while the lower part of his body is still busy copulating. Although I don’t know of any market studies of this interesting system in which trading of commodities involves male suicide and female cannibalism, I would anticipate that fat males do better business than thin ones, unless females haven’t had a meal in a while, in which case they probably mate with, and quickly devour, males of any size that happen to come their way.

  MARKETS FOR BUSINESS OTHER THAN SEX

  Believe it or not, not all animal business revolves around sex. Animals also trade in other commodities. In monkeys, grooming can be used as payment for sex, but it can also be traded for grooming itself or for other goods or services. In
many cases, low-ranking individuals offer grooming to high-ranking ones in exchange for being left in peace while eating, or for protection when they are under attack by their monkey enemies. Childless females also offer grooming to mothers to be able to take a close look at, touch, and briefly hold their baby, a highly valued commodity among primate females. Market effects have been demonstrated in all of these contexts. Primates make higher grooming payments when commodities are in short supply: low-ranking monkeys groom high-ranking ones for longer periods of time in order to be tolerated around food during periods of food shortage when compared to periods of food abundance; and females groom mothers for longer periods to be allowed to touch their infants when there are fewer infants in the group, compared to when there are many infants.

  Let’s take a closer look at the market in which grooming is traded for tolerance, protection, or support. In primates living in large groups, such as macaques, baboons, and vervet monkeys, social interactions within the group are regulated by nepotism and dominance. Family members associate with one another and exchange grooming more often than they do with nonkin. While monkeys groom their family members because they “love” and support them, grooming between nonkin generally has a business purpose. In this regard, high-ranking and low-ranking individuals can be thought of as two classes of traders that exchange commodities. High-rankers can offer low-rankers tolerance around food, protection from harassment by group members, and agonistic support in fights against others. Therefore, high-rankers are valuable and attractive social partners with which to do monkey business. Low-rankers are tolerated within a group probably because they help out when the group has to fight against predators or other groups. Low-rankers fight in the front lines during wars with other groups, and if a predator attacks a group, they are more likely to be eaten first. When the group is at peace, however, and low-rankers don’t have to risk their lives on behalf of everyone else, the only commodity they can offer high-ranking individuals in exchange for their tolerance and help is grooming.